It is known that circuit breakers and disconnectors, hereinafter referred to as a whole as switches, comprise an outer casing and one or more electrical poles, associated to each of which are at least one fixed contact and at least one mobile contact that can be coupled to/uncoupled from one another.
Circuit breakers of the known art moreover comprise control means that enable displacement of the mobile contacts, causing their coupling to or uncoupling from the corresponding fixed contacts. The action of said control means is exerted traditionally on a main shaft operatively connected to the mobile contacts so that, following upon its rotation, the mobile contacts are brought from a first operative position to a second operative position, which are respectively characteristic of a configuration of switch open and switch closed.
In the case of the switches for low currents (indicatively up to 800 A), and for modest voltages (indicatively up to 690 V) there exist solutions that cause the main shaft to coincide with the mobile contacts, giving rise to a rotating element made of insulating material capable of guaranteeing both dielectric separation between the phases and, of course, proper transmission of the movements and resistance to the forces involved. The rotating element is usually supported by structural parts of the outer casing of the switch that basically define areas of bearing with the rotating element itself. Switches of this type present considerable advantages, such as, for example, a limited number of parts and a limited overall encumbrance.
The indicative technical limits of 800 A and 690 V for the switches that make use of the rotating element derive from the fact that, beyond these thresholds, there would be required of the rotating element levels of performance in terms of electrodynamic and mechanical resistance that are scarcely compatible with structural materials of an insulating type that are to have competitive costs.
From a practical standpoint, the requirement of higher mechanical characteristics has partially been met by introducing metal reinforcement bars, passing through the rotating element itself. Metal reinforcement bars pose, however, problems of interference with the characteristics of electrical insulation between the poles. In practice, modest increases of mechanical performance are inevitably accompanied by further decay of the insulation.
Another road followed in the known art for bestowing upon the rotating element higher characteristics of electrodynamic and mechanical resistance is that of increasing the radial dimensions thereof, solutions of this second type tend, however, to introduce greater friction and to jeopardize the general efficiency of the switch.
A more advanced solution, described in the patent application No. BG2005A000026 enables extension of the use of the rotating element also to switches for currents decidedly higher than 800 A by introducing bearings that suspend the rotating element itself from the control members. In particular, the latter solution reduces the friction and prevents the stresses from being transmitted by the contacts to the rotating element directly onto critical areas of the switch, such as, for example, the joints of the containment means.
Even though the latter solution enables exploitation of the switch over a particularly extensive range of levels of performance, there remain in any case physical limits of use linked not so much to the rated current, as rather to the short-circuit conditions (for example, 45 kA to 690 V).
During a short circuit, there occur in fact a number of phenomena that expose the switch to particularly serious stresses. In the first place, the switch is called upon to withstand, albeit for a short time, extremely high currents. In the second place, the switch is called upon to interrupt the short circuit effectively. The capacity of the switch to withstand for short times currents that are much higher than the rated current is known as electrodynamic strength. The capacity of the switch to interrupt the short circuit is known as breaking power.
The limits of electrodynamic strength are a consequence, for example, of the so-called phenomena of electrodynamic interference between conductors that are close to one another traversed by current. Said electrodynamic interference presents both with electrical stresses, and hence thermal stresses, and with mechanical stresses. As is known, phenomena of electrodynamic interference are triggered both between conductors traversed by similar currents (such as, for example, between the various branches in parallel that form one and the same pole made up of a number of contacts) and between conductors that are close to one another traversed by different currents (such as, for example, between contiguous poles of a multiphase switch). In the case, for example, of similar conductors in parallel (as occurs between the various contacts of one and the same pole), considerable imbalance is encountered in the distribution of the current between the various contacts, also when the contacts have identical or similar morphological characteristics. For example, in the case of five conductors that are the same as one another, it is realistic to expect imbalance of a ratio of even in the region of three to one between the external conductors and the internal ones. In particular, in the case of short circuit the limits of electrodynamic strength will be reached rapidly by the external contacts that are subjected to higher electrical stresses.
The electrodynamic strength of a pole can thus be considered to a first approximation as the sum of the currents circulating in all the contacts of a pole as long as the outermost contacts remain in conditions of safety. In other words, it may be said that the various contacts do not contribute equally to form the electrodynamic strength of the pole.
The electrodynamic phenomena between conductors traversed by different currents are more complex because they derive from situations with a higher degree of variability, but in the ultimate analysis lead to further limitations of the electrodynamic strength in conditions of safety.
In particular, it may be noted that in multi-pole switches, since the external contacts of the individual poles are the ones subjected by the current to the higher stresses, the phenomena of interference between adjacent poles are in turn disadvantageously amplified.
It is known that the electrodynamic strength can be theoretically improved by increasing the distance between the electrical parts corresponding to contiguous poles, and/or using particularly strong contact springs, and/or by varying the geometry of the individual contacts. However, for the reasons already set forth, the modifications in this sense sooner or later come into conflict with dimensional constraints, with economic constraints on the cost-to-benefit ratio, and with the technical limits of the materials generally available.
Finally, not to be neglected is the fact that the electrodynamic interference presents also in the form of mechanical stresses, above all between different poles. It is necessary in fact to bear in mind that both the purely electrical parts of the pole and the various mechanical elements present in the neighbourhood and in the cavities of the rotating element can be variously traversed by electric currents. Along the electrical and kinematic chain of the pole, there are encountered in fact numerous metal elements and hence elements that conduct current (such as mobile contacts, springs, connecting rods, pins, flexible conductive elements) supported by and constrained both to one another and to the rotating element itself. In particular, said electrical and mechanical parts, if traversed by a component of the current of the pole, are exposed to mechanical stresses. Said stresses depend upon the currents involved, and in conditions of short circuit, the stresses produced can easily interfere with the limits of yielding and failure of the various materials. Excessive stresses can in fact cause mechanical seizing and failure both of the metal parts and of the plastic material that constitutes the rotating element. It is thus evident that also the mechanical phenomena deriving from the electrodynamic interference contribute to limiting the overall electrodynamic strength of the switch.
As regards the electromechanical parts, it should be pointed out that also momentary or limited mechanical deformations can easily jeopardize proper functioning of the switch.
As regards the shaped body of the rotating element, it should be recalled, instead, that, since it is an insulating material, the limit of yielding can be relatively modest, also when high-quality plastic materials are used, such as, for example, the so-called moulding compound with a base of unsaturated polyester.
It is clear that, if it is desired to achieve further increased performance for the switch (for example, with electrodynamic strength higher than 45 kA to 690 V), it would be necessary to be able to contain the electrodynamic stresses.